1. Field of the Invention
The present invention relates to a semiconductor device such as a high electron mobility transistor (HEMT; High Electron Mobility Transistor) employing nitride semiconductor materials as typified by GaN, and a manufacturing method of the semiconductor device.
2. Description of the Related Art
A conventional HEMT employing nitride semiconductor materials as typified by GaN has a structure where a thin AlGaN layer is deposited over a thick GaN layer and electrons are passed within the GaN in the vicinity of the AlGaN layer.
For example, in non-patent document 1, an AlN buffer layer (50 nm in thickness), a GaN layer (750 nm in thickness), and an AlGaN layer (30 nm in thickness) are deposited over a semi-insulating SiC substrate. Of those layers, only the AlGaN layer and the GaN layer in the vicinity of the AlGaN layer are required for operations, and the AlN layer and the GaN layer in large part are provided for making a high-quality crystal on the SiC substrate.
In general, an area where a current required for operations flows is called a “channel” and an area for improving the crystal property is called a “buffer.” In non-patent document 1, however, since most of those layers are buffers, those layers are called “buffers” without discrimination between the buffer and the channel.
Non-patent document 1 shows a relation between the drain current and the drain voltage in a GaN-HEMT which is constructed as described above. A large drain current exceeding 1 A/mm and a change in drain current achieved by a change in gate voltage are observed, and excellent electric characteristics are obtained therein.
Since an HEMT is in an off state when the gate voltage is smaller than or equal to the pinch-off voltage, the drain current does not flow under normal conditions. However, in the GaN-HEMT shown in non-patent document 1, when the drain voltage is increased, the drain current gradually increases. The drain current is a redundant leakage current that is not requisite in the transistor operation.
Non-patent document 1: Pei, Y. et al., “X- and Ka-band power performance of AlGaN/GaN HEMTs grown by ammonia-MBE”, Electronics Letters, Vol. 44, Issue 9, Apr. 24, 2008, pp. 598-598.
In conventional GaN-HEMTs, there is a problem that leakage current flows in the off state below the pinch-off voltage. The leakage current deteriorates the efficiency of a high power amplifier. Further, when the gate length is shortened, the leakage current increases, thus also interfering with the improvement of high frequency performance of the HEMT.
In order to investigate the cause of the leakage current, the inventors of the present invention performed simulations on a semiconductor device. As a result, the inventors found that the leakage current flows from the drain to the source by its passing through the lower portion of the GaN layer under the gate electrode. It is probable that since the gate voltage dominates the portion of the GaN directly under the gate electrode, no current flows in the portion; however, since the dominance of the gate voltage does not extend to the lower portion of the GaN layer, the leak current easily flows in the portion. For this reason, when simulations on a semiconductor device were carried out in the cases where the GaN layer was reduced in thickness, it turned out that the leak current thereof was reduced.
It was found that the reduction of thickness of the GaN layer is effective for improvement of the leak current as discussed above. However, merely reducing the film thickness of the GaN layer causes the reduction of the film thickness of the buffer layer for improving the crystal property, which causes a problem that the crystal property deteriorates. Therefore, conventionally, it is very difficult to reduce only the film thickness of the channel where electrons flow while keeping a state of the buffer layer having a large thickness.